Dual beam optical absorption photometry detector assembly for high pressure applications

ABSTRACT

A detector assembly providing a sample cell and a reference cell for use in a dual beam optical absorption photometry instrument wherein the reference cell and sample cell are formed by a pair of bores extending through a main detector body in an axial direction from one end of the body to the other end, each end of the body being provided with apparatus to seal the reference and sample bores from each other comprising a transparent lens with a flat surface positioned at the end of the body and over the cell openings, the outer face of the lens being convex, a thin sheet gasket material overlying the outer face of the lens, a metal pressure plate overlying the gasket material, both the gasket and pressure plate being provided with openings therein aligned with the sample and reference bores in the cell body, and means for exerting a pressure on the surface of the pressure plate whereby the pressure plate exerts a force on the gasket and lens to force the lens against the end wall of the cell body, the pressure plate deforming under pressure to assume the convex shape of the outer face of the lens whereby the pressure applied by the pressure plate is spread over the surface area of the lens with a maximum pressure applied at the central region thereof which is aligned with the wall section of the main body between the two cell bores. In one embodiment, an additional gasket material is positioned between the flat surface of the lens and the end of the cell body with suitable openings therein aligned with the sample and reference cell bores, this additional gasket material either being heat treated to fuse to the cell body and lens face and provide additional sealing or left in its natural state to provide resilience at the surface of the seal.

United States. Patent 1 3,795,450 Mar. 5, 1974 Munk PrimaryExaminer-William L. Sikes Attorney, Agent, or FirmStanley Z. Cole;Gerald M.

Fisher I p [57] ABSTRACT A detector assembly providing a sample cell anda ref- I erence cell for use in a dual-beam optical absorptionphotometry instrument wherein the reference cell and sample cell areformed by a pair of bores extending through a main detector body in anaxial direction from one end of the body to the other end, each end ofthe body being provided with apparatus to seal the reference and samplebores from each other comprising a transparent lens with a flat surfacepositioned at the end of the body and over the cell openings, the outerface of the lens being convex, a thin sheet gasket material overlyingthe outer face of the lens, a metal pressure plate overlying the gasketmaterial, both the gasket and pressure plate being provided withopenings therein aligned with the sample and reference bores in the cellbody, and means for exerting a pressure on the surface of the pressureplate whereby the pressure plate exerts a force on the gasket and lensto force the lens against the end wall of the cell body, the pressureplate deforming under pressure to assume the convex shape of the outerface of the lens whereby the pressure applied by the pressure plate isspread over the surface area of the lens with a maximum pressure appliedat the central region thereof which is aligned with the wall section ofthe main body between the two cell bores. In one embodiment, anadditional gasket material is positioned between the flat surface of thelens and the end of the cell body with suitable openings therein alignedwith the sample and reference cell bores, this additional gasketmaterial either being heat treated to fuse to the cell body and lensface and provide additional sealing or left in its natural state toprovide resilience at the surface of the seal.

10 Claims, 3 Drawing Figures ll-rm 1 LIGHT SOURCE v PATENTEDHAR 519M3.795.450

SHEEI 1 OF 2 SE SE DUAL BEAM OPTICAL ABSORPTION PHOTOMETRY DETECTORASSEMBLY FOR HIGH PRESSURE APPLICATIONS BACKGROUND OF THE INVENTION Dualbeam optical absorption photometry which employs the measurement of theoptical absorption properties of substances for qualitative andquantitative analysis is widelyused in the field of liquid columnchromatography wherein the effluent detector measures and records theconcentration of the separated solutes as they emerge from the column inthe eluting solvent. A dual cell detector is utilized having a first orsample cell for the sample comprising the solutes and eluting solvent asthe sample emerges from the column and a second or reference cell which,in one analysis method, contains the solvent alone, such that opticalabsorption due to the solvent in the reference cell can be subtractedfrom the reading obtained from the solvent and solutes in the samplecell to give a resultant reading of the sample under analysis.

Since such detectors are designed to measure very small changes insolute concentration, the sample cell and also the reference cell are ofsmall volume; to increase the sensitivity, the optical path through thesubstances is maintained long. This is accomplished by providingelongated, small diameter cell chambers. To eliminate readingdifferences introduced by differences in the light passing through thetwo cells, the cells are positioned very close together physically and asingle ultraviolet source is employedto produce the two separate opticalbeams through the sample and reference cells.

The above considerations dictate a detector assembly comprising anelongated cylindrical detector body with i the sample and referencecells formed by a pair of small diameter, parallel bores extending inthe axial direction through the body from one end to the other andspaced close together. Ingress and egress openings are provided for eachcell bore, these openings extending into the cylindrical body in adirection normal to the axis of the cell bores or passageways. Opticallytransparent plates such as lens and/or windows are pressed against thetwo ends of the cylindrical body to seal off the ends of the sample andreference cells. A single light source is then employed to direct afirst beam of light axially through the long, narrow sample cell andonto an associated detectorpositioned at the other end thereof and todirect a second beam of light axially through the similar shapedreference cell and onto a second separate I light detector at the otherend.

This assembly performs satisfactorily with the two cells sealed fromeach other so long as the pressure differential between the sample celland the reference cell is not great. However, at times it is desirablethat the sample detector assembly be able to withstand the full columnhead pressure of present day liquid chromatograph systems, for example5,000 PSI. This permits location of the reference cell downstream of theliquid pump and either upstream or downstream of the sample injectorstage but upstream of the column. Such a location provides a high degreeof compensation for changes in solvent composition. These changes may beeither accidental or, in the case of gradient elution, introduced onpurpose.

Where the reference cell is located upstream of the sample injectionpoint, it is exposed only to the solvent and changes in the lighttransmitted through the reference cell reflect only changes in thesolvent composition; these changes can be used to subtract out thesolvent component in the output sample from the column. The degree ofcompensation is high, although affected by the time interval betweenpassage of a given amount,

of solvent through the reference cell and through the detection cell. VI

In critical applications the reference cell can be located at the headof the column (or some intermediate point in the column) but below thepoint of sample injection. This location of the reference cell shortensthe time interval between passage of the liquid through the two cellsand therefore improves the degree of compensation for changes in solventcomposition. For example, as the injected sample passes through thereference cell it gives a negative response on the detector. Since thesample components have not been separated at this point, there is asingle response, and this response occurs prior to arrival of theseparated components in the detection cell at the output end of thecolumn. This reference peak is easily recognized due to its negativedeflection and early arrival. Such a peak provides a precise timeindicator for the measurement of the elution times of subsequentdetection cell indicated peaks. It also provides an integrated measureof the total amount of UV absorbing constituents in the sample.

Detector assemblies able to withstand high pressure differentialsbetween reference and sample cell are also needed in super criticalchromatograph, where the solvent is held above its critical temperatureand pressure throughout the separating column. The solvent pressure atthe exit of the column is likely to be a few thousand P.S.I., and theuse of troublesome flow restrictors before the solvent enters thedetector can be eliminated by the use of detectors able to withstandhigh pressures.

The major weakness of the detector cell when employed under highpressure differentials occurs at the two ends of the cells where thecells are sealed from each other by the transparent lens or windows.Although these transparent plates areheld against the ends of thedetector body under pressure exerted by screw-down means so as towithstand pressures of the order of 2,0003,000 PSI, the seal tends tobreak down as the pressure rises to the 4,0006,000 PSI region, and

leakage occurs between the sample and reference cells at one or bothends of the detector body.

SUMMARY OF THE PRESENT INVENTION The present invention provides a noveldual cell detector assembly wherein the transparent lens seal off thesample and reference bores at the ends of the detector body in a mannerwas to withstand high pressure differentials of the order of 5,0006,000PSI. In a preferred embodiment, each lens is held against the ends ofthe body by a suitable screw-down means acting through a thin, flexiblemetal pressure plate extending over the convex outer surface area of thelens. The pressure plate is deformed by the pressure action of thescrew-down means during assembly so as to substantially conform to theconvex shaped surface area of the lens. In this preferred embodiment, athin rubber-like gasket material is sandwiched between the lens and theing pressure to the outer face of the 3 flexible metal pressure plate tocushion the lens from the pressure plate. Both the pressure plate andthe gasket material are provided with openings therein aligned with theoptical path through the sample and reference cell passageways of thedetector body.

In applying pressure to the pressure plate to deform it against theconvex outer surface of the lens, a maximum pressure is exerted at thecentral portion of the lens to in turn exert a maximum pressure betweenthe transparent plate and the detector body at the wall region locatedat the center region of the body and between the sample and referencecells.

In a second embodiment of, the detector cell assembly a gasket materialis positioned between the end wall of the body and the lens to provide aresilience at the surface between the lens and the cell body. In oneinstance the gasket is left in its normal state. In another instance,while pressure is applied to the lens to force it against the gasket andbody, heat is applied to produce a fusion of the rubber-like gasketmaterial to the end wall of the body and to the lens. Thereafter, thisgasket acts to seal the ends of the two cells when the final assembly ismade with a screw-down means applylens.

BRIEF DESCRIPTION OF THE DRAWINGS DESCRIPTION OF THE PREFERREDEMBODIMENTS Referring now to the drawings, the novel dual cell detectorassembly of the present invention comprises a cell body 11 formed of astainless steel cylindrical block, the cell body being provided with apair of centrally disposed, spaced apart bores 12 and 13 forming thepassageways or cells for the sample and reference materials,respectively. The sample cell 12 is provided with a sample ingressopening or passageway 14 and associated conduit or tubing 15 directednormal to the cell 12 and a similar egress passageway 17 and conduit 18.In like manner, the reference cell 13 is provided with ingress andegress passageways 21 and 22 and associated conduits 23. Thus a samplesubstance may pass into the passageway 12 through the ingress tubing 15and passageway 14 and pass out from the cell through the egresspassageway 17 and associated tubing 18. In like manner, the referencematerial flows through the reference cell 13. These sample and referencepassageways 12 and 13 extend completely through the cylindrical cellbody 11 in the axial direction so that a first beam of light from acommon light source 24, e.g. UV light, may pass through the samplepassageway 12 and the sample contained therein and impinge upon a firstlight detector apparatus 25 at the opposite end of the detectorassembly. A second beam of light from the common light source 24 isdirected through the reference substance in the reference cell 13 andonto a second light detector 26 positioned at the light egress end ofthe detector assembly.

The cell body or chamber 11 is positioned in an axial bore 27 formed ina pair of mating annular body members 28 and 29, each body member beingprovided with cutaway sections 31, permitting the conduits 15, 18 and 23to pass in to the cell chamber 11. Each end of the cell chamber 11 issealed by apparatus which is of similar construction and only theelements on one of the two ends will be described in detail. Atransparent lens 34 is positioned over the end of the cell chamber 11and the openings 12 and 13 therein, the inner surface 35 of the lensbeing flat and the outer surface 36 being convex. A rubber-like discshaped gasket material 37 is positioned over the convex outer face 36 ofthe lens 34 and a stainless steel pressure disc 38 overlies the gasket.Both the gasket 37 and disc 38 are provided with a pair of openings 39therein which are aligned with the sample and reference cells 12 and 13in the cell chamber to permit the light beams to pass therethrough.During the initial assembly, the metal pressure disc 38 is flat andduring assembly assumes the concave shape shown in FIG. l.

An outer annular block member 31 is fixedly secured to the inner blockmembers 28 and 29 via several mounting bolts 40 utilized to clamp theouter and inner blocks together to form a solid integral body assembly.The outer annular block member 41 is provided with a threaded bore 42 inthe central region thereof. A threaded nut 43 is screwed into thethreaded bore 42 of the outer body 41 with the inner end of the .nutengaging an O-ring 44 which in' turn engages the peripheral region ofthe metal pressure disc 38. By turning the nut 43 so that it moves intoward the cell chamber 11, a force is exerted on the outer peripheralregion of the pressure disc 38 which causes the disc to be urged againstthe gasket 37 and convex surface 36 of the lens 34, the pressure disc'38 being deformed from its initial flat shape to the concave shapeshown in FIG. 1 and thus assuming the configuration of the outer face ofthe lens 34. g I

This deformed pressure plate therefore exerts a sig nificant pressure onthe lens 34 and urges it in a sealing manner against the cylindricalcell chamber 11, thus sealing the two cell passageways 12 and 13 fromeach other. Since the pressure disc 38 was initially flat and contactedlens 34 via the gasket 37 only at the central region of the lens 34, themaximum pressure is exerted at the central region, i.e. the regionbetween the sample cell 12 and reference cell 13 where the wallseparates the two cells.

A typical pressure disc 38 is stainless, 0.5 inch diameter and 0.02 or0.03 inch thick with the openings 39 having a diameter of about 0.08inch and spaced apart about one eighth inch. The gasket or cushioningdisc 37 is teflon (FEP) or rubber with a 0.05 inch diameter and 0.005inch thick; the openings therein are slightly larger than the openingsin the pressure disc, i.e. about 0.10 inch diameter.

A detector assembly constructed in this manner is able to withstandpressure differential between the reference and sample cell of 5,000 to6,000 PSI.

In a second embodiment of this invention (see FIG. 2), a thinrubber-like gasket material 32 such as FEP Teflon 0.005 inch thickhaving two openings 33 aligned with the reference and sample cellpassageways in the cell chamber is positioned between the inner face 35of the lens 34 and the end wall of the cell chamber 11. This gasketprovides aresilience at the surface of the seal and reduces therequirement for the degree of mechanical matching between the sealingsurfaces of the lens and cell body. To further enhance the sealingduring fabrication of this detector cell assembly and prior to theinsertion of the pressure screw means 43, a force is applied to theouter face 36 of the lens 34 whichin turn applies force to the thingasket material 32 positioned between the lens 34 and the cell body 11.Heat is then applied to the assembly to cause the gasket material tomelt and fuse to both the cellbody 11 and the lens 34 to thereby fixedlysecure the lens to the cell body and form a seal between the lens andthe body and seal off the area between the reference and cellpassageways. Thereafter, the pressure disc 38 and gasket 37 may beplaced over the outer surface 36 of the lens 34 and the screw means 43tightened down to urge the pressure disc 38 and gasket 37 against theouter face of the lens, thus deforming the pressure disc 38 as describedabove and exerting a constant pressure against the lens. This pressureis well distributed over the total surface area of the lens although amaximum pressure is present at the central region of the lens asdescribed above What is claimed is: l. A detector assembly for a doublebeam optical absorption photometer comprising i a cell chamber having apair of passageways extending therethrough from one end thereof to theother end, said passageways being spaced apart and parallel with a wallportion formed between said two passageways by said cell chamber, saidcell cham- Y ber-having an inlet passage and an outlet passage for oneof said passageways for passing a sample substance therethrough and aninlet passage and an outlet passage for the other of said passagewaysfor passing a reference substance therethrough, whereby opticalradiation may be passed in one beam through the sample substancepassageway from said one end thereof to said other end thereof andpassed in a second beam through the reference substance passageway fromsaid one end thereof to said other end thereof, 1 and sealing means ateach end of said cell chamber,

each sealing means comprising a lens positioned with a flat inner faceagainst the end of the cell chamber and over the two ends of the twopassageways and the wall section therebetween, the outer face of thelens being convex,

, and means for pressing said lens against the end of i said cellchamber to form a pressure seal between the lens inner face and the endof the cell chamber to seal the passageways from each other comprising adisc shaped sheet of flexible gasket material with an inner faceoverlying the convex face of said lens,

a metal pressure disc with an inner face overlying said flexible gasket,said gasket and said pressure disc each having a pair of openingstherein aligned with the pair of passageways in said cell.chamber toallow passage of the optical beams therethrough,

and means for applying pressure to said pressure disc whereby saidpressure disc and gasket conform to and press against the convex face ofsaid lens to sealably urge said lens against said cell chamber. 2. Adetector assembly as claimed in claim 1 wherein said pressure applyingmeans comprises an annular member engaging the outer face of saidpressure disc near the peripheral region thereof, and means for urgingsaid annular member against the peripheral region of said pressure disc.3. A detector assembly for adouble beam optical absorption photometercomprising a cell chamber having a pair of passageways extend.- ingtherethrough from one end thereof to the other end, said passagewaysbeing spaced apart and par allel with a wall portion formed between saidtwo passageways by said cell chamber, said cell chamber having an inletpassage and an outlet passage for one of said passageways for passing asample substance therethrough and an inlet passage and an outlet passagefor the other of said passageways for passing a reference substancetherethrough, whereby optical radiation may be passed in one beamthrough the sample substance in one passageway from said one end thereofto said other end thereof and passed in a second beam through thereference substance in the other passageway from said one end thereof tosaid other end thereof, and sealing means at each end of said cellchamber,

each sealing means comprising a lens positoned with an inner surfacefacing the end of the cell chamber and over the two ends of the twopassageways and the wall section therebetween,

a rubber-like gasket material sandwiched between the end of said cellchamber and said inner lens surface, said gasket having a pair ofopenings therein aligned with the pair of passageways in said cellchamber to allow passage of the optical beams therethrough,

and means for applying pressure to'the outer face of said lens to urgeit against the gasket and cell chamber end wall.

4. A detector assembly as claimed in claim 3 wherein the outer face ofsaid lens is convex and wherein said pressure applying means comprises adisc shaped sheet of flexible gasket material with an inner faceoverlying the convex face of said lens,

a metal pressure disc with an inner face overlying said sheet gasket,said sheet gasket and said pressure disc each having a pair of openingstherein aligned with the pair of passageways in said cell chamber toallow passage of the optical beams therethrough,

and means for applying pressure to said pressure disc whereby saidpressure disc and sheet gasket conform to and press against the convexface of said lens to urge said lens against said fused gasket and saidcell chamber.

5. A detector assembly as claimed in claim 4 wherein said pressureapplying means comprises an annular member engaging the outer face ofsaid pressure disc near the peripheral region thereof, and means forurging said annular member against the peripheral region of saidpressure disc.

6. A detector assembly as claimed in claim 3 wherein said gasketmaterial is fused to said cell chamber and said lens.

7. The method of sealing the flowing sample cell region from the flowingreference'cell region in the detector assembly of a double beam opticalabsorption photometer wherein the cell chamber has a pair of cellpassageways extending therethrough from one end thereof to the otherend, said passageways being spaced apart and parallel with a wallportion formed by said cell chamber between said two passageways, saidcell chamber having an inlet passage and an outlet passage for one ofsaid cell passageways for passing a sample substance therethrough and aninlet passage and an 7 outlet passage for the other of said cellpassageways for passing a reference substance therethrough, wherebyoptical radiation may be passed in one beam through the sample substancecell passageway from said one end thereof to said other end thereof andpassed in a second beam through the reference substance cell passagewayfrom said one end thereof to said other end thereof, and wherein eachcell end is sealed by a transparent'lens positioned with a flat innerface against the end of the cell chamber and over the ends of the twocell passageways and the wall section therebetween to form a pressureseal between the lens inner face and the end of the cell chamber toisolate the cell passageways from each other, the outer face of the lensbeing convex, comprising the steps of positioning a disc shaped sheet offlexible gasket material with an inner face overlying the convex face ofsaid lens, positioning a metal pressure disc. with an inner faceoverlying said flexible gasket, said gasket and said pressure disc eachhaving a pair of openings therein aligned with the pair of cellpassageways in said cell chamber to allow the optical beams to passtherethrough, and exerting pressure against the outer face of saidpressure disc to deform said pressure disc and said gasket to conform toand press against the convex face of said lens to sealably urge saidlens against said cell chamber.

8. The method as claimed in claim 7 wherein the step of exertingpressure against the outer face of said pressure disc comprises exertingsaid pressure against the peripheral region of the pressure disc.

9. The method as claimed in claim 8 including the step of l positioninga rubber-like gasket material between the end of said cell chamberandsaid inner lens surface, said gasket having a pair of openings thereinaligned with the pair of passageways in said cell chamber to allowpassage of the optical beams therethrough. 10. The method as claimed inclaim 9 including the step of heating said latter gasket material tofuse it to the end of the cell chamber and the inner face of said lens.

1. A detector assembly for a double beam optical absorption photometercomprising a cell chamber having a pair of passageways extendingtherethrough from one end thereof to the other end, said passagewaysbeing spaced apart and parallel with a wall portion formed between saidtwo passageways by said cell chamber, said cell chamber having an inletpassage and an outlet passage for one of said passageways for passing asample substance therethrough and an inlet passage and an outlet passagefor the other of said passageways for passing a reference substancetherethrough, whereby optical radiation may be passed in one beamthrough the sample substance passageway from said one end thereof tosaid other end thereof and passed in a second beam through the referencesubstance passageway from said one end thereof to said other endthereof, and sealing means at each end of said cell chamber, eachsealing means comprising a lens positioned with a flat inner faceagainst the end of the cell chamber and over the two ends of the twopassageways and the wall section therebetween, the outer face of thelens being convex, and means for pressing said lens against the end ofsaid cell chamber to form a pressure seal between the lens inner faceand the end of the cell chamber to seal the passageways from each othercomprising a disc shaped sheet of flexible gasket material with an innerface overlying the convex face of said lens, a metal pressure disc withan inner face overlying said flexible gasket, said gasket and saidpressure disc each having a pair of openings therein aligned with thepair of passageways in said cell chamber to allow passage of the opticalbeams therethrough, and means for applying pressure to said pressuredisc whereby said pressure disc and gasket conform to and press againstthe convex face of said lens to sealably urge said lens against saidcell chamber.
 2. A detector assembly as claimed in claim 1 wherein saidpressure applying means comprises an annular member engaging the outerface of said pressure disc near the peripheral region thereof, and meansfor urging said annular member against the peripheral region of saidpressure disc.
 3. A detector assembly for a double beam opticalabsorption photometer comprising a cell chamber having a pair ofpassageways extending therethrough from one end thereof to the otherend, said passageways being spaced apart and parallel with a wallportion formed between said two passageways by said cell chamber, saidcell chamber having an inlet passage and an outlet passage for one ofsaid passageways for passing a sample substance therethrough and aninlet passage and an outlet passage for the other of said passagewaysfor passing a reference substance therethrough, whereby opticalradiation may be passed in one beam through the sample substance in onepassageway from said one end thereof to said other end thereof andpassed in a second beam through the reference substance in the otherpassageway from said one end thereof to said other end thereof, andsealing means at each end of said cell chamber, each sealing meanscomprising a lens positoned with an inner surface facing the end of thecelL chamber and over the two ends of the two passageways and the wallsection therebetween, a rubber-like gasket material sandwiched betweenthe end of said cell chamber and said inner lens surface, said gaskethaving a pair of openings therein aligned with the pair of passagewaysin said cell chamber to allow passage of the optical beams therethrough,and means for applying pressure to the outer face of said lens to urgeit against the gasket and cell chamber end wall.
 4. A detector assemblyas claimed in claim 3 wherein the outer face of said lens is convex andwherein said pressure applying means comprises a disc shaped sheet offlexible gasket material with an inner face overlying the convex face ofsaid lens, a metal pressure disc with an inner face overlying said sheetgasket, said sheet gasket and said pressure disc each having a pair ofopenings therein aligned with the pair of passageways in said cellchamber to allow passage of the optical beams therethrough, and meansfor applying pressure to said pressure disc whereby said pressure discand sheet gasket conform to and press against the convex face of saidlens to urge said lens against said fused gasket and said cell chamber.5. A detector assembly as claimed in claim 4 wherein said pressureapplying means comprises an annular member engaging the outer face ofsaid pressure disc near the peripheral region thereof, and means forurging said annular member against the peripheral region of saidpressure disc.
 6. A detector assembly as claimed in claim 3 wherein saidgasket material is fused to said cell chamber and said lens.
 7. Themethod of sealing the flowing sample cell region from the flowingreference cell region in the detector assembly of a double beam opticalabsorption photometer wherein the cell chamber has a pair of cellpassageways extending therethrough from one end thereof to the otherend, said passageways being spaced apart and parallel with a wallportion formed by said cell chamber between said two passageways, saidcell chamber having an inlet passage and an outlet passage for one ofsaid cell passageways for passing a sample substance therethrough and aninlet passage and an outlet passage for the other of said cellpassageways for passing a reference substance therethrough, wherebyoptical radiation may be passed in one beam through the sample substancecell passageway from said one end thereof to said other end thereof andpassed in a second beam through the reference substance cell passagewayfrom said one end thereof to said other end thereof, and wherein eachcell end is sealed by a transparent lens positioned with a flat innerface against the end of the cell chamber and over the ends of the twocell passageways and the wall section therebetween to form a pressureseal between the lens inner face and the end of the cell chamber toisolate the cell passageways from each other, the outer face of the lensbeing convex, comprising the steps of positioning a disc shaped sheet offlexible gasket material with an inner face overlying the convex face ofsaid lens, positioning a metal pressure disc with an inner faceoverlying said flexible gasket, said gasket and said pressure disc eachhaving a pair of openings therein aligned with the pair of cellpassageways in said cell chamber to allow the optical beams to passtherethrough, and exerting pressure against the outer face of saidpressure disc to deform said pressure disc and said gasket to conform toand press against the convex face of said lens to sealably urge saidlens against said cell chamber.
 8. The method as claimed in claim 7wherein the step of exerting pressure against the outer face of saidpressure disc comprises exerting said pressure against the peripheralregion of the pressure disc.
 9. The method as claimed in claim 8including the step of positioning a rubber-like gasket material betweenthe end of said cell chamber and said inner lens surface, said gaskethavinG a pair of openings therein aligned with the pair of passagewaysin said cell chamber to allow passage of the optical beams therethrough.10. The method as claimed in claim 9 including the step of heating saidlatter gasket material to fuse it to the end of the cell chamber and theinner face of said lens.